How to implement lazy load for images?

Question

There is native lazy-load for images now on the web.

Native lazy-loading is coming to the web! https://t.co/LgF7F1iMgR defers offscreen images until the user scrolls near them. Shipping in Chrome ~75 https://t.co/4gR7lvx4zx pic.twitter.com/luCHEfLkKD

— Addy Osmani (@addyosmani) April 7, 2019

How would you implement lazy-load before this ?

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